DESIGN OP OPEN-HEARTH FURNACES 293
air on top of the jet of flame, and in this manner protected the
roof. As the coldest gases tend to seek the lowest portion of the
chamber, it is somewhat difficult to see just how this argument
is tenable. At the same time the waste gases reaching this large
port should be somewhat hotter than those reaching the smaller
gas-port at a lower level. This difference in temperature might
not be very great, as the vertical distance between the ports is not
great.
However, a much larger proportion of the waste gases would be
drawn off through this port, and, as a result, the air would be
preheated to a higher temperature than the gas. With this con-
struction the gas regenerator is strangled; that is, it cannot
obtain the proportion of the waste gases necessary to preheat the
incoming gas to the same temperature as the air supply, unless a
considerable draft differential is available to force a correct
division of the waste gases. A further disadvantage of this type
of head is the tendency of the bath to chill at the incoming end
with irregular reversals.
Port erosion, increasing the area of the gas-port and reducing
the jet velocity, has been very troublesome, as a point is speedily
reached at which the proper sintering of the bottom becomes
impossible. This entails shutting down the furnace for repairs
with consequent loss of production. Water-cooled ports are a
partial cure for the trouble; they increase the time between port
repairs, but they do not contribute to the correct division of the
waste gases between the regenerators.
Another result of the improper division of the waste gases
between the regenerators is a tendency for the furnace to work cold;
that is, the time per melt is increased, owing to the improper com-
bustion conditions produced. At the same time, the wear upon
the furnace is greater, particularly upon the roof, gas ports, cinder
line, etc.
The durability of any material exposed to heat is dependent
upon its ability to conduct this heat away from the heated end
and emit a sufficient amount of heat from its cool end to prevent
the hot end from overheating. When the hot end commences to
absorb heat faster than the cool end can emit heat, the temperature
of the hot end will commence to rise until it fails.
Water cooling supplies a more rapid method of removing heat
than air currents; it is particularly valuable in those cases where